An oxygen molecule can easily cross a plasma membrane while sodium (na+) ions are unable to cross the membrane without assistance. this is largely due to the fact that:

The hydrophobic core of the membrane does not allow the charged particles to pass through it.

Explanation:

The inner core of the membrane is made up of hydrophobic fatty acid tails of phospholipids. Due to this, some substances such as large, charged and polar particles cannot cross the membrane. This is why Na+ with a positive charge cross the membrane with the help of transport proteins only. On the other hand, molecular oxygen (O2) has a small structure and does not carry any charge. Therefore, it can cross the membrane without any help of transport proteins.

an organism's phenotype is the  form of a trait  that it  displays. for  flowercolor, a pea  plant  can  display  a phenotype of purple or white flowers. an organism's  genotype  is the alleles of a gene it contains. based on his data, mendel concluded that a phenotype can be  determined  by more than one  genotype.